Plasma and Fusion Research: Regular Articles

Volume 3, 031 (2008)

Electrostatic Potential Measurement by Using 6-MeV Heavy Ion
Beam Probe on LHD∗)
Takeshi IDO, Akihiro SHIMIZU, Masaki NISHIURA, Shinji KATO, Haruhisa NAKANO,
Akimitsu NISHIZAWA1) , Yasuji HAMADA, Mitsuhiro YOKOTA, Kiwamu TSUKADA,
Hideki OGAWA, Tomoyuki INOUE, Katsumi IDA, Mikiro YOSHINUMA,
Sadayoshi MURAKAMI2) , Kenji TANAKA, Kazumichi NARIHARA, Ichihiro YAMADA,
Kazuo KAWAHATA, Naoki TAMURA and the LHD Experimental Group
National Institute for Fusion Science, 322-6, Oroshi, Toki, Gifu, 509-5292, Japan1)
Tono Geoscience Centre, Japan Atomic Energy Agency, Toki, Gifu, Japan.
2)
Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan(Received 7 December 2007 / Accepted 8 April 2008)
A heavy ion beam probe (HIBP) using a 3 MV tandem accelerator was installed in Large Helical Device(LHD). It is designed to measure the electrostatic potential in the core region directly. It is calibrated and canbe used to measure the electrostatic potential profiles in LHD plasmas. The radial electric field (Er ) obtained
from the potential profiles measured using the HIBP agrees with that measured by charge exchange spectroscopy(CXS). Er predicted by the neoclassical theory is also compared to that measured using the HIBP, and is in good
agreement with the experimental results in the core region.
c 2008 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: heavy ion beam probe, HIBP, electrostatic potential, electric field
DOI: 10.1585/pfr.3.031

1. Introduction

using the HIBP is compared with that estimated by charge
exchange spectroscopy (CXS), which is another reliable
tool for determining Er by measuring the velocity of theplasma flow. Er calculated based on the neoclassical theoryis also compared with the results obtained using the HIBP.
In Sec. 4, the future prospects of fluctuation measurement
are discussed. The results are summarized in Sec. 5.

A heavy ion beam probe (HIBP) is a unique diagnostic tool for measuring the electrostatic potential, its fluctuations and density fluctuations in high-temperature magnetically confined plasmas directly without perturbing theplasmas. HIBPs have been installed in various types ofdevices [1–16] and useful results have been obtained.
In non-axisymmetric plasmas, the radial electric field(Er ) plays crucial roles in transport phenomena. For example, the electron confinement is improved in the coreregion during electron cyclotron heating (ECH) [17–22].
This is considered to be related to the transition from theion-root to the electron-root of Er predicted by the neoclassical theory, and called the Core Electron Root Confinement (CERC) [23]. Recently, meso-scaled structure of Er ,termed as zonal flow, is confirmed to exist in plasmas universally, and shown to aﬀect the radial transport of plasmas
by regulating turbulence. Therefore, understanding the behavior of Er and its eﬀects on transport is important forpredicting the performance of future fusion plasmas. To
achieve this, an HIBP has been developed for Large Helical Device (LHD) [24–26].
In this article, the apparatus of the HIBP in LHD (referred to as LHD-HIBP) is described in Sec. 2. The measured potential profiles are shown in Sec. 3. Er measured

2. LHD-HIBP System
2.1

System overview

In an HIBP, singly charged ions are injected into a
plasma as a primary beam, and doubly charged ions obtained by ionization due to collision with the plasma on
its path are detected (referred to as the secondary beam).
Since the energy diﬀerence between the primary and secondary beams is equivalent to the electrostatic potential energy at the position where the secondary beam is produced,
we can obtain the plasma potential locally by measuring
the secondary beam energy.
In order to extract the probing beam from the plasma,
the Larmor radius of the probing beam of the HIBP must
be comparable with the minor radius of the plasma device. The necessary beam energy (Eb ) is scaled approximately as Eb ∝ a2 B2 /mb , where a, B, and mb are the sizeof the plasma device, magnetic field strength, and massof the beam ion, respectively. Since the magnetic field in
LHD is approximately up to 3 T and a Larmor radius ofa few meters is necessary, singly charged gold ions (Au+ )author’s e-mail: ido@LHD.nifs.ac.jp∗) This article is based on the invited talk at the 24th JSPF Annual Meeting(2007, Himeji).

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Fig. 1 Schematic of the LHD-HIBP system.

with energies of approximately up to 6 MeV are requiredas the primary beam. Thus far, the 2-MeV HIBP in TEXTU was the highest energy HIBP available, and there weresome technical diﬃculties relating to high-voltage operation. Thus, in order to avoid diﬃculties, a tandem accelerator and a new type of energy analyzer have been adopted
in the LHD-HIBP. They are described in following subsection.
Figure 1 shows a schematic of the LHD-HIBP system. The primary beam (Au+ ) is injected from a lower
vertical port of LHD, and the secondary beam ionized in
the vacuum vessel (Au2+ ) is extracted from a horizontal
port and its energy is analyzed. The injection and detection positions are designed so as to measure the potential
at the magnetic axis in the standard magnetic configuration of LHD, in which the major radius of the magnetic
axis (Rax ) is 3.75 m, quadrupole magnetic field component(Bq ) is 100 %, and pitch parameter of the helical coil (γ) is
1.254.
The observation position can be selected by changing
the combination of magnetic field strength, beam energy,
and injection angles. Observable positions are shown in
Fig. 2. Actual observation positions are distributed in thetoroidal direction and the positions in the figure are projected on a poloidal cross section by tracing the magnetic
field line from the actual observation point. For example,
when the toroidal magnetic field strength (Bt ) is 3.0 T and
Rax is 3.75 m, the potential profile on the curve passing
through the magnetic axis can be measured by sweeping

Fig. 2 Observable region. The sample volume traces the marked
curve during a sweep of the probing beam. The sample
volumes of √the probing beam with the same Larmor radius (Lb ∝ Eb /Bt ) trace the same curve, where Eb and
Bt are the beam energy and the toroidal magnetic fieldstrength, respectively. (a) Rax = 3.6 m, Bq = 100%,
γ = 1.254. (b) Rax = 3.75 m, Bq = 100%, γ = 1.254.

a probing beam with an energy of 5.5 MeV. The lower
and upper sides of the observable region are limited by the
injection and extraction ports, respectively; thus the LHDHIBP can measure the potential in the core region of plas031-2

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2.3

mas.
Measurement with high temporal resolution is one of
the advantages of HIBPs. Currently, the temporal resolution of the LHD-HIBP is about 100 µs, which is limited by
the frequency response of the high-gain amplifiers (−3 dB
at 15 kHz). Although the frequency response is not sufficient to measure micro-turbulence, fast transitions due toelectric bifurcation and low-frequency MHD modes can bedetected marginally.
In the following subsections, some components of the
LHD-HIBP are described.

Beam transport system

The accelerator is installed in the basement of the
LHD building for convenient maintenance and to minimize
the influence of the stray magnetic field from LHD. However, this increases the length of the beam transport line forbeam injection becomes long (about 20 m).
Because the stray magnetic field deflects the beam
trajectory by several centimeters in the beam line, steering electrodes are installed to compensate the deflection atNos. 9 and 11 in Fig. 1.
Adjustment of beam focusing is essential for optimizing the size of sample volumes. Thus, electrostatic
quadrupole lenses are installed (No.10 in Fig. 1). They arearranged as a focusing-defocusing pair; hence, the focus of
the beam can be adjusted in two dimensions independently.
The beam profiles are measured with two-dimensional rotating wires shown as No.6 in Fig. 1. The beam divergencecan be estimated by changing the optical characteristics ofthe lenses and measuring the beam profile. In the experiments described in this paper, a beam with a diameter ofabout 10 mm at the injection port is adjusted to be focusedat the detector for the secondary beam, and the size of thesample volume is about < 40 mm in the minor radius direction [31], which determines the spatial resolution.

2.2 Ion source and 3 MV tandem accelerator
A MeV-range beam is required for the LHD-HIBP
as described above. To reduce the acceleration voltage, atandem accelerator is employed to obtain this high-energy
beam, unlike previous HIBPs in which single-ended accelerators have been used. The tandem accelerator includes anionizer, which is a gas cell for the LHD-HIBP, at the center,and high positive voltage is applied there. Singly charged
negative ions are initially injected into the accelerator andaccelerated toward the gas cell. They are ionized to positive ions by collisions with neutral particles in the gas cell.
Then, they are accelerated again toward a port on the otherside, which is grounded. Thus, the tandem accelerator canaccelerate the beam particles twice, and the required acceleration voltage can be reduced to half of that corresponding to the required beam energy. In the LHD-HIBP, theacceleration voltage is up to 3 MV and singly charged positive ions with energies of up to 6 MeV can be extracted.
The stability of the acceleration voltage, expressed in termsof root-mean-square deviations (RMSD), is 5 V for an applied voltage of 0.779 MV in a magnetic field of 1.5 T, and
250 V for an applied voltage of 2.635 MV in a magnetic
field of 2.829 T. Since the fluctuations of the energy of the
probing beam are double the above values, they should beconsidered in potential measurement, especially in strong
magnetic fields (> 2.0 T).
A tandem accelerator requires negative ions. Hence,
plasma-sputter-type negative ion sources have been developed since the development of the LHD-HIBP [27–29]. At
present, the output current is about 12 μA under the steady
operation. Since the conversion eﬃciency from Au− to
Au+ in the gas cell of the accelerator is ≤ 20 %, the currentof the primary beam is a few microamperes.
The energy spread of the negative ion beam from theion source is measured as 8 eV [28]. Since the negativeions are ionized in the gas cell installed in the tandem accelerator, the collisions with the neutral gas also cause energy spread of the beam. The energy spread in the gas cellis estimated to be ≤ 20 eV or less [30]. It will not aﬀect
potential measurement because the change in the average
beam energy is measured for the HIBP, and the number ofthe detected particles is suﬃcient for measurement of theaverage energy (≥ 1000 µs−1 ).

2.4

Octupole sweepers

The position of the sample volume can be selected bychanging the incident angle to plasmas. Unlike trajectoriesof the probing beam in axisymmetric devices, which aremostly on a poloidal plane, the probing beams in helicalsystems move toward the toroidal direction as well as ona poloidal plane. Thus, the incident angle must be controlled in two dimensions. In addition, the incident angleof the secondary beam to the energy analyzer can change
during a sweep of the probing beam, which possibly causes
errors in the analysis of beam energy. Therefore, in the
LHD-HIBP, two octupole sweepers are installed at the injection and extraction ports. The sweeper at the injectionand extraction ports serve to adjust the incident angles tothe plasma and the energy analyzer, respectively, and latter
one is to adjust the incident angles to the energy analyzer.
This method has been developed in an HIBP in the Compact Helical System (CHS) [32]. The sweep frequency of
the applied voltage is 10 Hz which is limited by the maximum current of the high-voltage amplifiers. Thus, the potential profile can be measured with a repetition frequencyof 20 Hz.
The octupole electrodes are horn shaped to deflect
the beam most eﬀectively. As a result, the electric fieldamong the electrodes has a three-dimensional structure.
Since the measurement positions of the HIBP are usually estimated only by trajectory calculations, the threedimensional structure of the electric fields should be considered in the trajectory calculation. In order to simulate
the beam trajectory precisely, the electric field is calculated
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Volume 3, 031 (2008)by the three-dimensional boundary element method. The
results of the unit-tests of the octupole sweeper confirm
that the calculation can simulate the electric field correctly.
The calibration of the trajectory of the probing beam in
LHD has been described in Ref. [33]. The results indicatethat the probing beam can be detected using the calculated
sweep voltages, although small deviations due to the straymagnetic field should be compensated.

2.5 Tandem energy analyzer
The change in the electrostatic potential in plasmas
is obtained from analysis of the change in energy of thesecondary beam. In traditional HIBPs, parallel plate electrostatic analyzers [34] have been used because they havethe desirable property of second-order focusing for the incident angle. However, this requires an impractical voltage of several hundred kV for the MeV-range beam of the
LHD-HIBP. Hence, we have devised a new energy analyzer with tandem electrodes [25, 35] to reduce applied
voltages and to maintain the second-order focusing for theincident angle available in the LHD-HIBP. The designed
incident angle of the traditional parallel plate analyzer is 30
degree, but the incident angles of the new tandem energy
analyzer are designed as 6 degrees for the first electrodeand 10 degrees for the second electrode. Consequently, therequired voltages can be reduced to 56.5 kV on the firstelectrode and 113.6 kV on the second electrode for a 6MeV beam.
The energy change (ΔEb ) is analyzed on the basis of
the displacement of the beam on the detector as ΔEb =−2.27 × 10−3 · Eb · Hslit · ΔND, where Eb is the energy ofthe primary beam, Hslit is the height of the opening of theentrance slit, and ND is the displacement normalized by
the height of the slit [25]. The height of the slit opening is
set to 3 mm in view of the intensity of the detected beamand the spatial resolution. Thus, the dynamic range of thepotential measurement is 6.81 × 10−3 · Eb .
Dependence of energy analysis on the incident angle
is undesirable for HIBPs because the incident angle canchange while the probing beam is swept to measure thepotential profile, which can cause errors in energy analysis. The incident angle dependence of energy analysis isshown in Fig. 3. The vertical axis is the displacement of thebeam on the detector, which is normalized by the height ofthe slit opening, and the beam energy is estimated fromthe normalized displacement. Since the geometry of theelectrodes in the beam line limits the incident angle to 0.5
degrees, the incident-angle dependence is acceptable forpotential measurement. The beam energy is analyzed withan accuracy of < 63 eV for a 1.5-MeV beam even if theincident angle fully changes.

Fig. 3 Results of calibration of the energy analyzer using a 30keV thallium beam. The horizontal axis is the change in
the incident angle from the designed incident angle (6 degrees). The vertical axis is the displacement of the beamon the detector and is normalized by the height of the
opening of the entrance slit. The slit height is 1 mm inthis calibration. The change in the beam energy is estimated from the change in the normalized position. Eachmarked line shows the incident angle dependence of theenergy analysis. The beam energy is shown in the legend
as the deviation from 30 keV.

HIBP system, which consists of the ion source, the accelerator, the beam line components, the magnetic field in LHD,
and the energy analyzer, is necessary for the potential measurement. In calibration experiments, the secondary beamionized by collisions with the neutral gas injected into thevacuum vessel is used. Since there is no electric field in
the vacuum vessel when the plasma is not produced, thesecondary beam energy is identical to the incident energyof the primary beam. Thus, it can be controlled by adjusting the primary beam energy. In LHD, a small amount of
neutral gas injected from the NBI port is available for calibration in every discharge.
Figure 4 shows the analyzed secondary beams energy
when the primary beam energy is varied from 4.258 MeV(−2.0 keV) to 4.262 MeV (+2.0 keV). In the experiment,
hydrogen gas is used as the fuel gas and the gas pressure
is in the range of 10−3 to 10−2 Pa in the vacuum vessel of
LHD. Figure 4 indicates that the change in beam energy
is analyzed successfully, and the results verify the linearityof the energy analysis.
In plasma experiments, a probing beam is deflectedby a poloidal magnetic field due to plasma current. The
deflection can cause two types of errors in potential measurement using an electrostatic energy analyzer. One isbased on the principle of the energy analyzer. Since thebeam energy is analyzed using the flight distance of thebeam in the energy analyzer, only the velocity component
in the plane containing the designed beam trajectory in theanalyzer can be analyzed. The plane is vertical in the LHDHIBP. Thus, if the beam is deflected in the horizontal(toroidal) direction by the poloidal magnetic field, a de-

2.6 Calibration for potential measurement
The magnetic field in LHD is also essential for the
HIBP operation. Therefore, calibration of the integrated
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Fig. 4 Comparison between the incident and analyzed beam energies. The horizontal and vertical axes are the incident
and analyzed beam energies, respectively. The energies
are expressed as the diﬀerence from 4.262 MeV.

crease in the analyzed beam energy is observed, which corresponds to the horizontal velocity. In the LHD-HIBP, thedistance between the center of the detector and the plasmaedge is 11 m and the width of the detector is 80 mm in thehorizontal direction, thus the horizontal angle of the detectable beam (α) is limited mechanically to 7.3 × 10−3
rad, even if the beam is assumed to have no width and divergence. The maximum error caused by the deflection is
ΔEb /Eb = 1 − cos2 α = 5.3 × 10−5 , which corresponds
to 80-eV change in the probing beam when the incident
energy is 1.5 MeV. The error is estimated under extreme
conditions; the horizontal movement of the beam on thedetector is less than 1/50 of the above assumption. Therefore, the deflection due to the poloidal magnetic field does
not aﬀect potential measurement.
The other error caused by deflection is due to the inadequate alignment of the analyzer. In the analyzer, thebeam is selected by rectangular slits. The dimensions of
the slit opening are 3 mm in height (the electric field direction in the analyzer) and 100 mm in width (horizontal
direction). The width of the slit is wider than that of thesecondary beam, so that the beam can be detected even if
it is deflected by the poloidal magnetic field. In principle,
the horizontal displacement of the beam should never affect energy analysis. However, if the axes of the slit anddetector are not aligned with adequate accuracy, horizontal
movement causes an error in energy analysis. Hence, we
examined its influence by the sweeping the beam horizontally with the octupole sweeper shown as No.14 in Fig. 1.
Figure 5 (a) shows the displacement of the beam on thedetector during a horizontal sweep, and indicates that thedisplacement in the horizontal deflection is linear with thechange in the sweep voltage. The displacement in the vertical direction, with which the energy change is analyzed,
is small but finite. This suggests that the deflection in

Fig. 5 (a) Normalized displacement of the beam during a horizontal sweep. (b) Relationship between horizontal andvertical displacements.

the horizontal direction causes an error in energy analysis.
The vertical displacement (VD) is related to the horizontal(HD) as VD = −0.10 HD (Fig. 5 (b)). Therefore, this error
must be considered in potential measurement.

3. Potential Profile Measurement
3.1

Experimental condition

Experiments for comparison between the HIBP and
CXS were performed in a magnetic configuration where
Bt = 1.5 T, Rax = 3.6 m, Bq = 100%, and γ = 1.245. Hydrogen is used as the fuel gas. The energy of the probing
beam of the HIBP is 1.562 MeV, and the sample volume
traces on the curve shown in Fig. 6 (a) during a sweep of
the beam. Figure 6 (b) shows the timing of NBI and ECH,
and temporal evolution of the line averaged electron density. The plasma is produced and sustained by tangential
NBI of 3.8 MW, and ECH of 1.1 MW is superposed from
1.5 to 2.1 s. Perpendicular NBI of 1.2 MW is modulated tomeasure the profile of the poloidal flow by CXS.
The probing beam of the HIBP is swept at a frequency of 10 Hz. The secondary beam current is shownin Fig. 6 (c). The secondary beam is detected before theplasma production (t < 0.7 s), and data for calibration using the secondary beam is obtained. The temperature anddensity profiles are shown in Fig. 6 (d) and (e).
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Fig. 6 (a) Observation position (red curve) and the magnetic flux
surface (black dots). The sample volume traces the red
curve during a sweep of the probing beam. (b) Timing of NBI and ECH, and temporal evolution of the line
averaged electron density. (A) and (B) indicate periods for which the potential and Er profiles are shown.
(c) The secondary beam current. The beam is swept
at a frequency of 10 Hz. (d) Electron and ion temperature profiles measured by Thomson scattering and
CXS. (e) Density profiles reconstructed from line averaged density measured using an FIR interferometer.

Fig. 7 Error analysis of the HIBP signal. (a) Stability of the acceleration voltage. (b) Stability of the analyzer voltage.
(c) Eﬀect of the toroidal displacement. The horizontal
axis is the position of the sample volume which is projected on a horizontally elongated poloidal cross-section
by tracing the magnetic field line from the actual observation point.

might cause an error, but it is estimated to be < 10 eV [24]because the plasma density is low.
In previous HIBPs, the radiation from plasmas caused
errors occasionally because it produced photoelectrons in
the beam line, which caused drops in the sweep and analyzer voltages and pseudo signals in the detectors. In theexperiments described in this paper, however, no signal dueto the radiation is observed while the probing beam is notinjected. Thus, the radiation does not aﬀect the signal.
Therefore, the error in potential measurement is estimated to about 340 V.

3.2 Error estimation
First, we estimate the errors in potential measurement.
Possible error sources are fluctuations of the acceleration
and analyzer voltages, and electric noise of the amplifier.
The error due to electric noise of the amplifier depends
on the intensity of the detected beam. The ratio of the noise
to the beam intensity is about 0.025 in the core region of
the plasma, which corresponds to an error of about 340 V
in potential measurement.
The fluctuation in the acceleration voltage is shownin Fig. 7 (a). The maximum fluctuation in the acceleration
voltage is 80 eV, which is caused by a voltage-drop in thecommercial power source due to the operation of the heating apparatuses. The influence of this fluctuation can beremoved from the detected signal.
The fluctuation in the analyzer voltages is shown in
Fig. 7 (b), and is less than the noise level (2.5 V RMSD),
which corresponds to 0.25 kV RMSD change in beam energy. It is less than the noise level of the amplifiers.
The toroidal deflection due to the change in thepoloidal magnetic field is shown in Fig. 7 (c). The maximum of error in potential measurement is 50 eV, and theerror can be corrected in the potential profiles. The lossof beam energy through collisions with plasma particles

3.3

Comparison among HIBP, CXS, andneoclassical calculation

The potential profiles measured using the HIBP are
shown in Fig. 8 (a). During superposition of ECH (1.7 s),
the potential varies in the positive direction. The gradientof the potential increases especially in the region of ρ. >0.4 m, which indicates that a positive Er is formed there.
In Fig. 8 (b), Er obtained from the derivative of the potential profile measured using the HIBP is compared withthat estimated from the poloidal flow measured by CXS.
Although Er is not obtained by CXS for ρ < 0.6 because
CXS depends on the emission profile and the emission
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profiles can be measured successfully across whole radiusof the plasma by complimentary use of HIBP and CXS.
In a helical system, Er can be predicted by the neoclassical theory. Hence, Er profiles calculated using theMonte-Carlo simulation code DCOM [36] are also compared with those measured using the HIBP in Fig. 8 (c).
The calculated Er agrees with that measured by HIBP. The
calculation predicts the transition from the ion root to electron root at ρ ∼ 0.6 during the NBI phase. Er measured
using the HIBP also changes from weakly negative to positive at about ρ = 0.6. The results indicate that the DCOM
code simulates the Er profiles measured using the HIBP in
the core region, although there is a diﬀerence between theexperimental and calculation results in the edge region; thereason for this is not yet clear.

4. Possibility of Fluctuation Measurement
One of the advantages of HIBPs is high temporal resolution measurement. Thus, measurements of potential anddensity fluctuations have been attempted [37].
Figure 9 shows the temporal behaviors of the line averaged density, heating power, intensity of the secondary
beam of the HIBP (which indicates the density fluctuation in plasma), its frequency spectrum, and the potential
signal. The secondary beam current is usually detectedwith a set of four split plates. In this shot, however, only
one of the four split plates was connected to an amplifier with high frequency response (∼ 100 kHz). Thus, thepotential and density fluctuations cannot be separated in
the frequency range > 10 kHz. The measurement positionis near the plasma center, but the fluctuation of the secondary beam intensity possibly includes fluctuations along
the beam trajectory; this is called the path integral eﬀect.
According to a simulation, the magnitude of the eﬀect
depends on the correlation length of the fluctuation [38].
Thus, we do not discuss the locality of the fluctuation here.
In Fig. 9 (c), coherent fluctuations of the beam intensity appear in the frequency range of 15 to 30 kHz. One
fluctuation exists from 0.45 to 0.52 s with a frequency of
about 20 kHz and normalized amplitude of 1.8 %. The
other coherent fluctuation appears at 0.72 s; its frequency
increases rapidly from 15 to 25 kHz and then decreases
gradually to 21 kHz, and its normalized amplitude is 0.6%. The tendency of the change in the frequency of the latter fluctuation seems to correlate with the behavior of thepotential. The correlation may indicate that the frequencychange reflects a change in the Doppler shift due to the
E × B flow. The measurement of Er (potential profile) is
necessary to investigate the behavior of the fluctuation.
Thus far, we have detected fluctuations with normalized amplitude of about 0.6 %, and the noise level of thepower spectrum is about 0.4 %. Improvement of the S/Nratio is necessary to measure turbulent fluctuation in thecore region. After the above-mentioned experiments, we

Fig. 8 (a) Potential profiles measured using the HIBP. The horizontal axis is the same as that in Fig. 7 (c). “Gas” is a reference profile measured after the shot. Thus, it indicatesthe potential of the vacuum vessel of LHD. (b) Comparison of E r profiles measured using the HIBP and CXS. (c)Comparison of E r profiles measured using the HIBP andcalculated by the DCOM code based on the neoclassicaltheory. The latter profiles are labeled “DCOM”.

from the outside of the observed position contaminates thesignal in this phase, Er measured using the HIBP agrees
with that by CXS at ρ ∼ 0.6. The results indicate that Er
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in helical plasmas.

Acknowledgments
We are grateful to Drs. A. Fujisawa and H. Iguchi of
NIFS and Professor K. Ishii of Tsukuba University for useful discussions, and Professors O. Motojima, S. Sudo and
K. Itoh for continuous encouragement. We thank Professor T. Watanabe for accurate calculation of the magnetic
field in LHD. We thank the LHD technical group for theirsupport.
This work has been supported by MEXT Japan under Grant-in-Aid for Young Scientists (16760674 and
18760640) and by NIFS under NIFS04ULBB505, 507,
and 508.
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[38] H. Nakano et al., Proc. 17th international Toki Conference(2007).

Fig. 9 (a) Temporal evolution of line-averaged electron densityand electron temperature in the plasma center. (b) Secondary beam intensity of HIBP. (c) Temporal evolutionof the potential signal measured using the HIBP and timing of NBI and ECH. (d) Power spectrum of the fluctuation in the potential signal. Note that the fluctuation
with the frequency of > 10 kHz may be contaminatedwith density fluctuations.

have increased the primary beam current a few times byimproving the gas cell in the accelerator and the eﬃciencyof beam transport. The detection eﬃciency was also increased by 60 % and the noise in the amplifier was reduced.
The output of the negative ion source could be increased byincreasing the discharge current in the ion source. Therefore, precision of the fluctuation measurement will be improved in the near future.

5. Summary
An HIBP using a 3-MV tandem accelerator has been
installed in LHD and is calibrated as an integrated system
to measure the electrostatic potential in plasmas.
Er measured directly using the HIBP agrees with that
estimated by CXS near the half radius of the plasma. Since
the HIBP can access the core region where CXS has difficulty measuring Er and CXS can access the edge region
where HIBP has diﬃculty measuring Er , Er profiles can bemeasured across the entire radius of the plasma by combining the HIBP and CXS measurements. Therefore, thecomplementary measurement involving both the HIBP and
CXS and high temporal resolution measurement using the
HIBP will advance the study of the physics of Er formation
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